Oregon City resident Adrien Chase has been missing since Dec. 5

"He had a very nice birthday" on Dec. 5, when he turned 35, and shared cake with Geirlich around 11:30 that night. After she went to bed, Chase apparently left the house without his cell phone or jacket and likely walked a block to Atkinson Park, where he often took evening strolls.

But Chase, who suffers from bipolar disorder he controls with medication, did not return. His disappearance makes no sense to Geirlich, with whom he's lived for six months.

"This is very unusual," she said. "He was doing well, and he even wrote in his journal that he was looking forward to the New Year."

Family members filed a missing persons report with Oregon City Police on Dec. 7, and soon after, officers found Chase's newly filled prescriptions inside his backpack at Geirlich's house on Harrison Street.

"We believe he probably doesn't have his medications with him," said Sgt. Matt Paschall of OCP.

On Monday, after several hours combing Atkinson Park for clues to Chase's whereabouts, searchers went home without answers. "We found nothing," said Geirlich.

Police had already searched the park and several other locations Chase was known to frequent, according to Paschall, but have turned up nothing in Atkinson Park, along the railroad tracks and several cliff-side locations in the city. They also followed up on a reported sighting of Chase in the Southeast Portland on Monday after receiving a report he may be in the area of 122nd Avenue and Powell Boulevard in the company of unidentified homeless people.

Family friend Kari Bittner, who lives in Rochester, N.Y., and is helping to coordinate search activities from the East Coast along with Chases mother, Joellen Kuhl, fears for the man she babysat when he was a toddler.

Adrien left home Dec. 5 and no one has heard from him since, Bittner said Monday morning. If he fell in Atkinson Park or had some other accident there well, 10-plus days is a very long time to be in this weather without a jacket.

Paschall said information about Chase has been listed on national missing persons databases and that officers would continue to follow leads  including a tip from a woman saying she had spotted Chase in southeast Portland on Dec. 13.

"She told us that she fed him and that he could be hanging out with transients," said Paschall. "We're looking into that, and we certainly hope we can find him safe."

Since Chase had a habit of taking evening walks in the park at 1601 Jackson St., Geirlich and Bittner think he could have fallen victim to the terrain.

The park has a steep ravine and Abernethy Creek runs through it, noted Bittner. We are concerned that he could have fallen in that creek and been unable to get himself out.

We need to rule out the park ... one way or the other.

Family members grew more alarmed over the weekend because Chase had not been in contact with anyone for a week and a half, something thats uncharacteristic for him, Bittner said.

Meanwhile, Geirlich said she had hired a private detective from Washington state to look for her cousin. "Adrien didn't want to be lost," she said.

Police believe Chase was wearing cream-colored jeans and dirty, white, size 9 tennis shoes when he left his house 13 days ago. Chase is 5-foot-6 with a thin build and short, dark hair. Anyone with information about this case should call Oregon City Police tip line at 503-496-1616.